STABLE ISOTOPIC EVIDENCE FOR THE PEDOGENIC ORIGIN OF CALCITIC ROCKS OF ANDAMAN NICOBAR ISLANDS, BAY OF BENGAL, INDIA

Carbon and oxygen isotopes and electron microprobe analyses were carri ed out in some calcitic outcrops of the Andaman-Nicobar group of islan ds. Isotopic results in these rocks indicate significant depletion in deltaC-13 (-12.0 to -15.02 parts per thousand vs PDB) and low deltaO-1 8 values (18.42 to 23.32 parts per thousand vs SMOW). The presence of chromite, magnetite, quartz and chlorite relicts in the microprobe ana lyses and other geological features suggest that they were formed by t he alteration of ultramafic rocks. Brecciation and cementation of the ultramafics under the influence of meteoric water are attributed to th e formation of these calcitic lenses.